1. Field: This invention relates to apparatus employable in mineral processing. More specifically, the invention is directed to an apparatus for use in processing ores and concentrates through a technique known conventionally as bioleaching.
2. State of the Art: Bioleaching utilizes the chemical action induced by a species of autotrophic bacteria, e.g. thiobacillus ferrooxidans and thiobacillus thiooxidans, to process mineral ores and concentrates which are not otherwise efficiently treatable by other conventional methods, such as cyanide leaching. Various approaches involving bioleach based technology are treated in the literature. "Growth Kinetics of Thiobacillus Ferrooxidans Isolated from Arsenic Mine Drainage", Joan Forshaug Braddock, Huan V. Luong and Edward J. Brown, Applied and Environmental Microbiology, July 1984, pg. 48-55;"Continuous VAT Biooxidation of A Refractory Arsenical Sulphide Concentrate", a paper presented at the Seventeenth Canadian Mineral Processors Conference January 22-24, 1985 by P. Brad Marchant; South African Patent application No. 853701, "Bioleaching Process" by Albert Bruynesteyn, U.S. Pat. No. 2,829,964 (Zimmerley); and U.S. Pat. No. 4,571,387 (Bruynesteyn, et al.). For example, leaching of manganese from manganese ores is described in U.S. Pat. No. 3,433,629 (Iman et al.). A common bioleach approach has employed the use of a cascade system wherein a plurality of processing tanks are arranged in series. The mineral ores being processed are initially introduced into a slurry and then directed from tank to tank during the process whereby the initial substrate concentration is successively reduced to the final desired level in the last tank. A continuous process involving two or more tanks and a means of selectively controlling the level of sulfide within one or more of those tanks is described in "A New Process for Refractory Gold/Silver Concentrates", by R. P. Hackl, F. Wright and A. Bruynesteyn. In this approach, the user may obtain enhanced reaction rates by manipulating the quantity of oxidizable material upon which a generally captive biomass population is permitted to feed.
The chemistry operative in the bioleach process has been explored extensively in the art. Reference is made to "Ferrous Iron Oxidation and Uranium Extraction by Thiobacillus Ferrooxidans", by Roger Gray and Marvin Silver, Biotechnology and Bioengineering, Vol. 19, pages 727-740 (1977); "Use of Micro-Organisms for Recovery of Metals", by O. H. Tuovinen and D. P. Kelley, International Metallurgical Reviews, Review 179, 1974, Vol. 19, pp. 21-31; European Patent Application No. 0004431 of Interox Chemical and Microbiological Mining "Scientific American", Vol. 247(2), pg. 44 (1982) by Corale L. Brierly, which are hereby incorporated by reference.
Previous efforts in providing structures suitable for containing mineral-bearing ores and concentrates have generally been directed to cylindrical tank-like structures. Illustrative of these prior devices are those shown in U.S.S.R. No. 800221 (Ust-Kamenogorsk) and U.S.S.R. No. 899119 (Zhdanov Yul). Manufacturing techniques, together with material and cost considerations, have resulted in structures which are somewhat limited in their ability to process large quantities of ore efficiently and economically. Not only is the requisite processing equipment expensive, but furthermore, the energy required to operate this equipment oftentimes renders it less than cost beneficial.
Operationally speaking, bioleaching requires a generally continuous infusion of oxygen into a slurry which contains the ore being processed, carbon dioxide, water, a species of autotrophic bacteria, and nutrients, e.g. nitrogen, phosphorus, and magnesium for those bacteria. The provision of oxygen to adequately supply the resident bacterial population has previously made necessary the use of energy intensive machinery, e.g., turbines.
In a conventional approach, gas injection nozzles are fitted to a turbine fitted sparger. The nozzles inject oxygen-containing gas into the slurry proximate the rotating blades of a turbine. The blades mix the gas with the surrounding slurry, thereby distributing that gas throughout the slurry. Though mechanically simple in concept, the use of these nozzle/turbine arrangements does involve some disadvantages. First, the energy requirements of such turbines are often so excessive as to render their usage economically unfeasible. Secondly, with increasing tank diameter, turbine mixing becomes unfeasible.
Due to the relatively small surface-to-volume ratio of the coarse gas bubbles produced by the sparger/turbine arrangement, and owing to the disparate specific gravities of the gas bubbles and the slurry, these bubbles oftentimes rise through the slurry and are exhausted to the environment before the oxygen contained therein has been efficiently assimilated by the resident bacteria. The optimal condition appears to be the provision of oxygen in the form of fine bubbles which appear to possess an enhanced capability for assimilation by the bacteria. An increased surface-to-volume ratio and the accompanying enhanced assimilation characteristics of fine bubbles render fine bubbles the preferred alternative over coarse gas bubbles, which for an equivalent mass of air have significant lower surface to volume ratios and assimilation characteristics. Various attempts have been made to achieve the optimal fine bubble aeration condition.
Ust-Kamenogorsk in U.S.S.R. patent No. 800221 suggests the use of a central support having a plurality of outwardly extending radial arms affixed thereto. On the end of each arm is mounted a gas injection nozzle. The nozzles are held rigidly in place by their respective arms so gas (e.g. air) under high pressure may be passed therethrough into a surrounding slurry. The Ust-Kamenogorsk patent relies on the turbulence created by the gas flow to agitate the slurry and thereby retain the solids within that slurry in suspension.
The apparatus disclosed in U.S. Pat. Nos. 4,732,608 and 4,728,082 (Emmett Jr. et al.) relies on a slow moving diffuser arrangement wherein a plurality of upright porous membrane diffusers are rotated within the reactor vessel about a vertical axis. The suspension of the ore and concentrate solids in the slurry is in part achieved in the arrangement by an airlift which draws slurry upward through a hollow, centrally positioned shaft. Upon reaching a selected height above the surface level of the slurry body, the drawn slurry is distributed out over the body of the slurry.
The aforesaid structures are, due to the tank-like nature of their containment vessel, their oxygen introduction systems and their methods of maintaining the mineral-bearing solids in suspension, of necessity limited in the quantity of ore which can be processed over a given time interval. There continues to be a need for an apparatus which is suited for large-scale processing of mineral-bearing solids by means of a bioleaching method whereby the method's oxygen requirements are satisfied while avoiding the process rate limitations engendered by the apparati presently in use.